An electronic device may use a power amplifier and an antenna to transmit wireless communication signals. The power amplifier and the antenna can be designed to have matching impedances that enable power to be efficiently transferred from the power amplifier to the antenna without significant losses. The impedance of the antenna, however, can vary due to beamforming operations or a proximity of a user to the antenna. Consequently, the impedances of the power amplifier and the antenna can become mismatched and some power may be reflected back to the power amplifier. The reflected power can cause the power amplifier to experience decreased linearity or decreased power efficiency. In some cases, the reflected power may permanently damage the power amplifier.